We saw the var statement in Chapter 3, Variables and Data Types; it provides a way to explicitly declare a variable or variables. The syntax of this statement is:
var name_1 [ = value_1] [ ..., name_n [= value_n]]
That is: the var keyword is followed by a variable name and an optional initial value, or it is followed by a comma-separated list of variable names, each of which can have an initial value specified. The initial values are specified with the = operator and an arbitrary expression. For example:
var i; var j = 0; var x = 2.34, y = 4.12, r, theta;
The var statement should always be used when declaring local variables within functions. Otherwise, you run the risk of overwriting a top-level variable of the same name. For top-level variables, the var statement is not required. Nevertheless, it is a good programming practice to use the var statement whenever you create a new variable. It is also a good practice to group your variable declarations together at the top of the program or at the top of a function.
Note that the var statement can also legally appear as part of the for and for/in loops, in order to declare the loop variable as part of the loop itself. For example:
for(var i = 0; i < 10; i++) document.write(i, "<BR>"); for(var i = 0, j=10; i < 10; i++,j--) document.write(i*j, "<BR>"); for(var i in o) document.write(i, "<BR>");